Organic amides, e.g., dimethylformamide (DMF), are widely used as solvents for the preparation of polymer solutions, such as poly(iminoimidazolidinediones), polyparabanic acids, polyamides, polyimides, polyurethanes, polyhydantoins and the like.
In U.S. Pat. No. 3,758,577 granted Sept. 11, 1973, to Dinbergs, the vagaries of DMF, including residual dimethylamine, were believed to result in degradation of polymer solutions in DMF. Dinbergs discovered DMF and polymer solutions thereof were stabilized with halogen-containing compounds of the structure ##STR1## wherein R is a saturated aliphatic hydrocarbon chain containing one to six carbon atoms, or an aliphatic hydrocarbon chain containing two to six carbon atoms and having a terminal double bond; R.sub.1, R.sub.2 and R.sub.3 are hydrogen or an alkyl group containing from one to four carbon atoms; and X is a halogen selected from the group consisting of chlorine, bromine and iodine.
It has now been discovered that the problem as perceived by Dinbergs was not fully characterized or appreciated. Dimethylamine has been determined to cause rapid polymer degradation, however, it is believed that not only residual dimethylamine, but also dimethylamine which is liberated by the slow hydrolysis of DMF by small quantities of water, contribute to the polymer degradation. Moreover, organic amides, in general, hydrolyze in the presence of water to liberate amines. Therefore, polymers dissolved in organic amides undergo the same type of degradation observed in DMF polymer solutions.
By way of example, polymers which are sensitive to amines include polyparabanic acids, poly(iminoimidazolidinediones) polyurethanes, polyureas, polyesters, polyamides, polyimides, polycarbonates and polymers which contain one or more of the groups characteristic of these polymers.
Organic amide solvents, e.g., DMF, are not only extremely difficult to dry, but absorb water when exposed to a humid atmosphere. It has been found that, depending on the method of preparing polymer solutions and the precautions taken to exclude moisture, the water concentrations may be high, e.g., 8 to 10% by weight.
Other methods which have been employed in the prior art to stabilize DMF include contacting the solvent with ion-exchange resins and molecular sieves prior to use.
The hydrolytic instability of DMF and other amide solvents and the resultant degradative effect of their hydrolysis products is manifested by molecular weight degradation of the dissolved polymer. Fibers and films made from solutions in which the polymer has undergone degradation are inferior to products which would be expected from the virgin polymer.
The instability of polymer solutions dissolved in organic amide solvents can be readily determined from the decline in viscosity of the polymer solution. Similarly, residual organic amide solvent in product polymer films, fibers, casting and the like can also cause polymer degradation during aging apparently by the same mechanism.
Thus, it is a feature of the present invention that a novel class of stabilizers has been found which, when dissolved in organic amide solvents, having in solution therein polymers which are subject to amine degradation, will inhibit degradation of the polymers dissolved therein. An additional advantage of the present stabilizers is the stabilization of polymer products produced from the stabilized solutions. These and other advantages and features of the present invention will be seen in the following descriptions.